WHAT IS a CV Axle?
CV (constant-velocity) axles, (also known as half-shafts), are used in front-wheel drive vehicles to transfer the engine’s power from the transaxle to the two drive wheels. Some late model vehicles with independent rear suspension and some four-wheel and all-wheel drive vehicles also use CV axles.
A CV axle has two CV joints (an inner joint and an outer joint). These joints allow the axle to transfer the engine's power to the drive wheels at a constant speed while accommodating for the different travel conditions, including the up and down motion of the suspension and cornering. Without constant-velocity joints, the axles would bind or break whenever the vehicle hit a bump in the road or turned a corner.
CV joints are packed with grease for lubrication. To keep the grease in the joint and moisture, dirt and roadway grime out, the joint is covered with a rubber boot called a CV boot. CV boots are made of a durable rubber that can withstand extreme weather and travel conditions. Good quality CV boots can easily last 100,000 miles or more. But, because they are constantly exposed to the elements, a CV joint boot can be damaged at any time. For this reason, it is a good idea to routinely inspect your CV boots for damage. If your vehicle has more than 125,000 miles - it is a good idea to check the CV joints more often.
Actually, CV axles rarely fail - it's the CV joint(s) that fail and they typically only fail because of a damaged or torn CV boot. When a CV boot fails due to age and wear, or is damaged by roadway debris, the grease that keeps the joint lubricated quickly leaks out of the joint and dirt, roadway grime and moisture enters the joint. When this occurs, the CV joint will soon fail.
Of the two joints, the outer CV joint almost always fails first because it does more work than the inner joint so it wears out sooner. The outer joint is also much more likely to be damaged from roadway debris because of its location. For these reason, if you suspect CV-joint failure, odds are it is the outer joint that has failed. (See above image)
NOTE: While a damaged or torn CV boot is almost always to blame for a failed CV joint, like any part, CV joints can wear out over time. As the joint ages and wears, tight tolerances give way to added space opening up between the moving parts of the joint. As space between the parts increases the parts begin to bang against each other and after awhile they break.
Symptoms of a Bad or Failing CV Joint
A bad or failing CV joint will oftentimes make a clicking, popping or grinding noise, especially when turning. At low speed, a bad CV joint may make a steady knocking sound every couple of seconds or so. A bad CV joint can also cause a vibration that is felt in the steering wheel and floorboard of the vehicle - this may happen only at certain speeds.
INSPECTING a CV Axle and Checking/Testing a CV Axle
A routine inspection of the CV axles and CV boots and checking/testing a CV axle you suspect of being bad are two different processes. Below is an explanation.
Routine CV Axle Inspection
Inspecting a CV axle involves a visual check of the CV boots. In most instances, all you will need is a flashlight.
Follow these Steps...
Checking/Testing a CV Axle You Suspect of Being Bad
Checking and testing a CV axle you suspect of being bad or beginning to fail involves a visual inspection, a movement test and a drive test.
Follow these Steps...
BEFORE replacing a CV boot, you must first determine the condition of the CV joint.
When the grease is expelled out of a CV joint due to a damaged or torn CV boot, the joint will begin to wear much faster and will soon fail. Once the CV joint is damaged, packing the joint with new grease and installing a new CV boot will not repair the already damaged CV joint. The only remedy for a damaged CV joint is replacing the CV axle. So, before repacking a CV joint with new grease and installing a new CV boot, you must first determine the condition of the CV joint.
How to Determine the Condition of a CV Joint
1. Using a bright flashlight, look inside the damaged or torn CV boot. Look closely at the CV joint - is there still some grease in the joint?
2. Firmly grasp the CV axle near the joint and attempt to move the axle back and forth and then in and out. There should be little or no movement in any direction.
3. Drive the vehicle to an empty parking lot or any open space where you can drive in a circle without endangering yourself or others. When safe to do so, turn the steering wheel all the way in one direction and driving in a circle. Now, do the same thing in the opposite direction. Did you hear a clicking or popping noise when traveling in a circle in either direction?
Conclusion: If there is still some grease in the CV joint, if you felt little to no movement when attempting to move the axle by hand and if there was no clicking or popping noise when driving in a circle in either direction then the CV joint is good. Replacing the damaged CV boot may be the appropriate repair.
NOTE: A CV joint that is dry (no grease) but passed the movement test and driving test is also good. However, the window of opportunity for packing the CV joint with new grease and replacing the CV boot is short. With no grease in the joint, joint failure will occur very quickly.
Types of CV Replacement Boots
There are basically three types of replacement CV boots. These are explained as follows:
The first type is a conventional CV boot design, which requires the CV axle to be removed from the vehicle and then the CV joint to be removed from the axle before packing the joint with grease and installing the CV boot. This is the best of the three CV boots but is also the most costly (or time consuming) to install.
The second type is a "stretch boot" design, which does not require the removal of the CV axle to install, but does require the removal of the wheel and wheel hub assembly. With this design, the CV boot is stretched over the CV joint and is held together using a special glue or large clamps. Because the stretch boot is a one piece boot, it may last longer than a split type boot but the downside is that installation is not as easy as the split boot design.
The third replacement CV boot is a "split-boot" design. Being split, this CV boot is installed by simply opening up the boot at the split and placing it over and around the CV joint. The boot uses a special glue or large clamps to seal the boot tightly around the CV joint. The split-boot design is installed without removing the wheel, hub or any other parts. Of the three boot designs, the "split-boot" is considered the weakest but is clearly the easiest and least costly boot to install.
CV Boot Replacement Cost
Auto repair shops and car dealership service centers do not typically replace CV boots - they will insist on installing a new CV axle whenever a CV boot and/or CV joint has failed. The reason is that the labor cost for replacing a CV boot is more than the cost for replacing a CV axle. The CV boot kit costs less than a CV axle but the difference is less than the labor difference. The example below illustrates the cost difference between replacing a CV boot versus a CV axle. The example vehicle is a 1999 Toyota Avalon.
VEHICLE: 1999 TOYOTA AVALON
REPAIR: Replace Outer CV Boot Driver's Side Axle
PARTS COST: $32
LABOR COST: 2.8 Hrs. X $85/hour = $238
TOTAL COST: $270
VEHICLE: 1999 TOYOTA AVALON
REPAIR: Replace CV Axle Driver's Side
PARTS COST: $137
LABOR COST: 1.5 Hrs. X $85/hour = $128
TOTAL COST: $265
In this example, the cost is $5 less to replace the CV axle versus just replacing the CV boot.
CV Boot Replacement Do-it-Yourself Cost
Consider the above, the only time it makes economic sense to replace a CV boot versus the CV axle is when you do the work yourself. CV boot kits can be purchased at your local auto parts store or online. The cost for a CV boot kit can range from a low of around $8 to as much as $40, depending on the vehicle you drive and the quality of boot kit you purchase. A CV boot kit comes with a CV Joint Boot, CV Joint Grease Pack and Glue or Clamps.
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